Recently, an HBT (Heterojunction Bipolar Transistor) has been aggressively researched and developed as an electronic device for the next generation to be incorporated in high output amplifiers for use in satellite communication due to its high speed characteristics and high current driving force. However, sufficient reliability has not yet been achieved for high output. One of the causes for insufficient reliability is a deterioration of the electrical characteristics in a p-type base layer doped with impurities of high concentration. For example, an HBT of npn-type comprises an n-GaAs collector layer, a p-GaAs base layer, and an n-AlGaAs emitter layer. Be is conventionally employed for doping the p-GaAs base layer by MBE (Molecular Beam Epitaxy). Conventional practices also comprise the use of materials, such as DEZ (diethylzinc) to form the p-GaAs base layer by MOCVD (Metalorganic Chemical Vapor Deposition), for doping the p-GaAs base layer with Zn.
It has been found, however, that Zn and Be are easily diffused in GaAs to such an extent that Zn and Be are diffused in the entire n-AlGaAs emitter layer during formation of the emitter layer or during testing after assembling the device, whereby the concentration of impurities is lowered in the emitter layer, eventually resulting in reduction of the collector current. To overcome such a drawback, carbon has recently become the focus of attention as a p-type dopant, because it is not readily diffused. Accordingly, the concept of a carbon (C)-doped GaAs base layer has arisen for improving the reliability of an HBT.
For example, the Journal of Crystal Growth 135 (1994) 629-632, H. Kohda et al. discusses the possibility of doping GaAs with carbon of high concentration employing trimethylarsenic (TMAs) and trimethylgallium (TMGa) in a MOCVD method.
Japanese Laid-Open Patent Publication (unexamined) No.203520/1990 discloses a method of growing a C-doped GaAs layer, employing TMAs and TMGa as raw materials, by heating a GaAs substrate to 650.degree. C. under an atmosphere of AsH.sub.3, stabilizing the pressure, introducing TMAs in addition to AsH.sub.3, and further introducing TMGa.
Japanese Laid-Open Patent Publication (unexamined) No. 190467/1993 discloses a method of growing an AlGaAs layer having a carbon additive amount of not less than 4.times.10.sup.19 cm.sup.-3 by establishing a substrate temperature of 450.degree. C. to 550.degree. C., employing a supply ratio of raw material gas TMAs/(TMGa+TMAl) of not more than 3, and growing at atmospheric pressure.
As for a semiconductor device different from an HBT, the Journal of Applied Physics (76), Jul. 1, 1994 590-592, R. B. Bylsma et al. reported a trial production of a C-doped InGaAs/AlGaAs laser.
There are several methods of growing C-doped GaAs. According to the report in the Journal of Crystal Growth 147 (1995) 256-263, N. Watanabe et al., GaAs growth during MOCVD has been researched in detail, and growth of GaAs doped with highly concentrated C may be easily achieved by employing TMAs as a Group-V raw material, employing TMGa as a Group-III raw material, and optimizing the growth conditions. The resulting p-GaAs layer is obtained by controlling the introduction of C from the raw materials TMAs and TMGa instead of employing an independent dopant. It is, however, reported that, even when forming an HBT having a p-GaAs base layer doped with C, there is a deterioration of the device characteristics due to hydrogen contamination, whereby hydrogen combines with the highly concentrated C dopant in GaAs.
For example, the Journal of Crystal Growth 145 (1994) 420-426, H. Fushimi et al. discusses the activation of holes by annealing, after acknowledging systematic inactivation of holes due to hydrogen concentration of high concentration C-doped GaAs. H. Fushimi et al. showed that the hydrogen contamination of C-doped GaAs layer brings about carbon inactivation.
Furthermore, in the preparatory printed speeches for the 42nd Applied Physics Association Lecture Meeting (1995) 1226, it was reported that hydrogen inactivation of carbon results in deterioration of the characteristics of a p-n junction diode.
Thus, the reliability of an HBT cannot be improved simply by adopting a C-doped GaAs base layer, because carbon is inactivated due to hydrogen contamination of the C-doped GaAs layer. Accordingly, there exists a recognized need to prevent such hydrogen contamination at the time of forming the C-doped GaAs layer.
Conventional MOCVD methods for the formation of an HET comprises:
(1) mounting a semi-insulating GaAs substrate on a wafer susceptor in a MOCVD furnace, increasing the temperature of the GaAs substrate under an atmosphere of arsine (AsH.sub.3), forming a predetermined GaAs layer when required, introducing TMGa serving as a Group-III raw material in addition to arsine and disilane (Si.sub.2 H.sub.6) as an n-type dopant at a prescribed flow rate at a predetermined temperature, thereby forming an n-GaAs collector layer;
(2) stopping the introduction of TMGa as a Group-III raw material and disilane as an n-type dopant, and changing a temperature of the wafer susceptor to a predetermined temperature under an atmosphere of arsine;
(3) introducing TMGa as a Group-III raw material and TMAs as a Group-V raw material at a prescribed flow rate when reaching the predetermined temperature, thereby forming a p-GaAs base layer doped with a high concentration C-doped p-GaAs base layer on the n-GaAs collector layer;
(4) supplying arsine simultaneously with stopping the introduction of TMGa and TMAs, and changing the temperature of wafer susceptor to a predetermined temperature under the atmosphere of arsine;
(5) introducing TMGa and TMAl as Group-III raw materials and disilane as an n-type dopant material at a prescribed flow rate when reaching the predetermined temperature, thereby forming an n-AlGaAs emitter layer on the mentioned n-AlGa base layer; and
(6) cooling the resulting semiconductor layer to a predetermined temperature in an atmosphere of arsine, after forming a prescribed GaAs contact layer when required.
In the conventional HBT p-GaAs base layer doped with highly concentrated carbon formed as described above, the semiconductor layer is both formed and cooled in an atmosphere of arsine. During such forming and cooling, atomic hydrogen from arsine is taken up in the p-GaAs base layer thereby inactivating carbon, eventually causing deterioration of the characteristics of a p-n junction diode.